fix(web): reveal the caret in the composer's own scrollport on a session switch

preventScroll stopped the browser from revealing a focus nobody gestured for, and took the caret with it. The composer DOM is reused across sessions, so switching to a longer draft keeps the previous offset while the value swap puts the caret at the new draft's end: measured on all three engines, 940px below a box sitting at 0, where the old geometry reached the caret at 628. The unlock effect now suppresses the browser's walk and reveals the caret in its own scrollport, landing at 625 of 628. The focus-keeping mousedown gets preventScroll too and needs no reveal. revealCaret returns early when the box cannot scroll: the draft fits, so every caret is already in view.

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -25,6 +25,8 @@ afterEach(cleanup)
 const ZERO_RECT = (): DOMRect => ({ top: 0, bottom: 0 }) as DOMRect
 Range.prototype.getBoundingClientRect = ZERO_RECT
 
+const nativeSetStart = Range.prototype.setStart
+
 const SCTX = {} as ClientContext
 const SID = 's1' as SessionId
 
@@ -323,8 +325,24 @@ describe('running and lock semantics (queue cut 1)', () => {
     const mirror = view.container.querySelector<HTMLElement>('[data-input-mirror]')!
     expect(mirror.firstChild).toBeInstanceOf(Text)
     scroll.getBoundingClientRect = () => ({ top: 100, bottom: 436 }) as DOMRect
+    // jsdom reports scrollHeight === clientHeight for every element, which is
+    // the composer's own "nothing to reveal" case; a scrollable box is what
+    // puts the reveal on the table at all.
+    Object.defineProperty(scroll, 'clientHeight', { value: 336, configurable: true })
+    Object.defineProperty(scroll, 'scrollHeight', { value: 964, configurable: true })
     Object.defineProperty(scroll, 'scrollTop', { value: 0, writable: true, configurable: true })
-    onTestFinished(() => { Range.prototype.getBoundingClientRect = ZERO_RECT })
+    onTestFinished(() => {
+      Range.prototype.getBoundingClientRect = ZERO_RECT
+      Range.prototype.setStart = nativeSetStart
+    })
+    // Which layer the caret is measured against, and at which index: the stub
+    // records `setStart` so a helper that measured the backdrop instead, or
+    // always collapsed at 0, fails here rather than only in the browser lane.
+    let measured: { node: Node; offset: number } | null = null
+    Range.prototype.setStart = function setStart(node: Node, offset: number): void {
+      measured = { node, offset }
+      nativeSetStart.call(this, node, offset)
+    }
     const caretAt = (top: number): void => {
       Range.prototype.getBoundingClientRect = () => ({ top, bottom: top + 24 }) as DOMRect
     }
@@ -336,6 +354,10 @@ describe('running and lock semantics (queue cut 1)', () => {
     fireEvent.paste(textarea, { clipboardData: { getData: () => 'pasted' } })
     await settle()
     expect(scroll.scrollTop).toBe(88) // 524 - 436
+    // Measured on the mirror's own text, at the index the paste left the caret
+    // (an empty draft's selection start, 0, plus the pasted length).
+    expect(measured!.node).toBe(mirror.firstChild)
+    expect(measured!.offset).toBe('pasted'.length)
     // A caret already inside the box does not move it.
     caretAt(200)
     fireEvent.paste(textarea, { clipboardData: { getData: () => 'more' } })
@@ -348,6 +370,29 @@ describe('running and lock semantics (queue cut 1)', () => {
     expect(scroll.scrollTop).toBe(48) // 88 - (100 - 60)
   })
 
+  it('a session switch refocuses without moving the transcript, and reveals the new draft caret', () => {
+    // The composer DOM is reused across sessions, so the previous session's
+    // offset survives while the value swap puts the caret at the new draft's
+    // end. `preventScroll` keeps the browser from revealing it through the
+    // conversation scrollport, which leaves the reveal to the effect itself.
+    const { view, textarea, props } = bench({ draft: 'line\n'.repeat(40) })
+    const scroll = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
+    const mirror = view.container.querySelector<HTMLElement>('[data-input-mirror]')!
+    onTestFinished(() => { Range.prototype.getBoundingClientRect = ZERO_RECT })
+    scroll.getBoundingClientRect = () => ({ top: 100, bottom: 436 }) as DOMRect
+    Object.defineProperty(scroll, 'clientHeight', { value: 336, configurable: true })
+    Object.defineProperty(scroll, 'scrollHeight', { value: 964, configurable: true })
+    Object.defineProperty(scroll, 'scrollTop', { value: 0, writable: true, configurable: true })
+    Range.prototype.getBoundingClientRect = () => ({ top: 500, bottom: 524 }) as DOMRect
+    const focused: (boolean | undefined)[] = []
+    textarea.focus = (options?: FocusOptions) => { focused.push(options?.preventScroll) }
+    textarea.setSelectionRange(textarea.value.length, textarea.value.length)
+    act(() => { view.rerender(<InputBar {...props} sessionId={'s2' as SessionId} />) })
+    expect(focused).toEqual([true])
+    expect(scroll.scrollTop).toBe(88) // 524 - 436
+    expect(mirror.firstChild).toBeInstanceOf(Text)
+  })
+
   it('disabled state shows the unavailable placeholder; custom placeholder wins', () => {
     const { textarea } = bench({ disabled: true })
     expect(textarea.placeholder).toBe('会话不可用')